1. Field of the Invention
The claimed invention relates to a method for tightening a bolt or a machine screw to exert a predetermined tension force. Many machines and other devices used in large industrial or smaller consumer applications are assembled with bolts which should be tightened to exert a predetermined tension force. For example, cylinder heads of internal combustion engines are usually anchored to the engine block by a pattern of bolts, each of which should be tightened to exert a predetermined, uniform, tension force. Steel building structures are also frequently assembled with bolts which should be tightened to exert a predetermined tension force. Pressurized fluid containing vessels and chemical reactors are often assembled by an arrangement of bolts intended to exert predetermined holding forces.
In such bolt assembly applications it is important that the predetermined tension force to be exerted be accurately achieved. If, for example, the cylinder head is not uniformly and tightly bolted to the engine block in an internal combustion engine, the engine head or underlying gasket may be damaged or leaks can occur. Similarly, steel building structures which are assembled with improperly or inaccurately tightened bolts will not achieve their specified strength and may be subject to fatigue and consequent weakness. In general, if the bolts are not tightened up to the predetermined specified tension force, they are too loose and may cause failure by vibrating looser or by allowing leakage of pressurized fluids or chemicals or by allowing metal parts to creep out of position. If the bolts are tightened too much, they become over-stressed and can fail by sudden rupture, for example, by the head breaking off from the shank or by the shank breaking at the threaded region. This type of over-stressed breakage can lead to sudden failure of the equipment involved, such as when the over-tight stud bolts shear off to release the wheels from a moving vehicle.
The present invention is intended to be used to properly and accurately tighten bolts, machine screws, threaded studs and the like in these and other applications.
2. Description of the Prior Art
A variety of devices have previously been used to determine certain characteristics of a tightened bolt. For example, U.S. Pat. No. 3,306,100 -- Wilhelm et al. discloses an ultrasonic bolt tension tester in which the resonant frequency of the bolt and changes in resonant frequency of the bolt as it elongates are measured. A very complex system is utilized involving the mixing of signals from a reference oscillator. U.S. Pat. Nos. 3,354,705 -- Dyer and 3,440,869 -- Hardiman disclose the use of strain gauges to measure the torque force being exerted on the head of a tightened bolt.
Power-driven torque wrenches which stop automatically when a desired applied torque is exerted on the head of a bolt are disclosed in U.S. Pat. Nos. 2,756,622 --La Belle and 3,429,179 -- Bowen et al. U.S. Pat. No. 2,600,549 -- Ledbetter discloses a torque wrench driven by an electric motor.
Other devices for measuring torque applied to the bolt head are disclosed in U.S. Pat. Nos. 2,957,342 -- Hanneman; 3,285,057 -- De Zurik; 3,643,501 -- Pauley; 2,968,943 -- Statham; 3,209,177 -- Minasian; 3,303,694 -- D'Onofrio; 3,486,369 -- Korzilius; 3,565,193 -- Wirth; and 3,368,396 -- Van Burkleo et al.
Devices of the type generally discribed in the patents noted above are not entirely satisfactory. Those previous devices which seek to measure the tension force exerted by a bolt by measuring the bolt's resonant frequency as an indication of its elongation are extremely complex. Others which attempt to measure the torque applied to the head of a bolt as an indication of tension force exerted actually do not measure torque of the bolt per se, but are influenced by a number of other factors. That is, this technique of measuring the torque applied to the head of a bolt really is rendered inaccurate and misleading by other effects such as the friction occuring between the head of the bolt and the washer or plate underlying the bolt head, the friction existing between the shank of the bolt and the bore hole and friction between the threads of the bolt and the threads in the bore hole. Thus, prior art devices which measure torque applied to the head of a bolt are partly measuring friction effects and not torque. If the bolt threads are rusty or dirty, the friction is high and the bolt is really not screwed up very tightly when the rated torque is applied to the bolt head. If the bolt is new and well greased, it is relatively easy to over-torque the bolt and twist its head off. The friction effects are mostly removed and the application of rated torque to the head may speedily twist the head to a point which exceeds the torsional strength of the shank. Thus the apparent torque applied to the bolt head is not a true indication of the tension force ultimately exerted, which is the quantity of actual interest.
Several prior art devices are capable of measuring applied torque to the bolt head, bolt elongation, or the tension force exerted by the bolt only after the bolt is installed. This two-step installation and subsequent testing operation undesirably increases installation time. An operator of such subsequent testing apparatus must be familiar with its operation in addition to the operation of the torque wrench or other device for installing the bolt in the first place.
In summary, prior art methods and apparatus for attempting to tighten bolts to exert a predetermined tension force have exhibited certain drawbacks.